Attention Deficit Disorder (ADD), a commonly diagnosed nervous system illness in children, is generally treated with methylphenidate hydrochloride (available commercially as, e.g., Ritalin.RTM.. Symptoms of ADD include distractibility and impulsivity. A related disorder, termed Attention Deficit Hyperactivity Disorder (ADHD), is further characterized by symptoms of hyperactivity, and is also treated with methylphenidate hydrochloride. Methylphenidate drugs have also been used to treat cognitive decline in patients with Acquired Immunodeficiency Syndrome (AIDS) or AIDS related conditions. See, e.g., Brown, G., Intl. J. Psych. Med. 25(1): 21-37 (1995); Holmes et al., J Clin. Psychiatry 50: 5-8 (1989).
Methylphenidate exists as four separate optical isomers as follows: ##STR1##
wherein R.sub.2 is phenyl. Pharmaceutically acceptable salts are generally administered clinically. Other phenidate drugs, which also can be administered according to the invention, include those in which the methyl group in the above structures is replaced by C.sub.2 -C.sub.4 alkyl and those in which R.sub.2 is optionally substituted with C.sub.1 -C.sub.4 alkyl.
Clinically, the threo pair of enantiomers of methylphenidate hydrochloride is generally administered for the treatment of ADD and ADHD. The hydrochloride salt is commonly referred to simply as "methylphenidate". Unless indicated otherwise, the term "methylphenidate" is used broadly herein to include methylphenidate and pharmaceutically acceptable salts thereof, including methylphenidate hydrochloride.
The threo racemate (pair of enantiomers) of methylphenidate is a mild central nervous system stimulant with pharmacological activity qualitatively similar to that of amphetamines. Undesirable side effects associated with the use of the DL-threo racemate of methylphenidate include anorexia, weight loss, insomnia, dizziness and dysphoria. Furthermore, the racemate, which is a Schedule II controlled substance, produces a euphoric effect when administered intravenously or through inhalation or ingestion, and thus carries a high potential for abuse.
Srinivas et al. studied the administration of DL-threo-, D-threo, and L-threo-methylphenidate to children suffering from ADHD, and reported that the pharmacodynamic activity of DL-threo-methylphenidate resides in the D-threo isomer (Clin. Pharmacol. Ther., 52: 561-568 (1992)). While DL-threo-methylphenidate is generally used therapeutically, this racemate includes the L isomer which apparently makes no significant contribution to the pharmacological effectiveness of the drug. The removal of the L isomer is expensive, however, and there has been no reason to do so.
An additional problem is that, generally, children being treated with dl-threo methylphenidate must take one or more doses during the day in order to receive optimal benefit from the treatment. This creates a problem for school administrators who must store a controlled substance on school premises, with the associated risk that it may be stolen for illicit use. Furthermore, children may be traumatized by ridicule from peers when they must take medication at school.
Sustained release formulations of DL-threo methylphenidate have been developed, which provide for slow release of the drug over the course of the day. However, it has been observed that peak plasma concentrations of the drug are lower when sustained release formulations are used as compared to conventional dosage forms administered throughout the day. In some studies, sustained release formulations of DL-threo methylphenidate have been shown to have lower efficacy than conventional dosage forms.
Pulsed-release dosage forms, wherein a single dosage form contains two doses, one of which is released shortly after ingestion and the other of which is released following a delay of several hours, have recently been proposed as a method for administering a maximally effective dose regime. While pulsed dosage forms provide for efficient release of multiple doses of medication at predetermined intervals, such dosage forms can be complex and expensive to manufacture. Furthermore, while pulsed-release dosage forms are suitable for administration of medications such as methyl phenidate to children, multiple releases of the medication are not required for all patients. However, it is desirable to administer to all patients the most effective and efficient dosage of mediation and, in the case of methyl phenidate, it is now believed that this end is best achieved by administering the single, effective isomer, i.e. D-threo methylphenidate.
While the D-threo isomer of methylphenidate has been shown to be the pharmacodynamically active isomer, the administration of the single isomer has been neither studied nor administered clinically on a chronic basis. Thus, the effects of administering a single isomer on a chronic basis as compared to the conventionally administered racemate have not heretofore been recognized or understood.
There remains a need for methods for delivering methylphenidate with maximum effectiveness and minimal potential for abuse. Furthermore, there is a need for a dosage form which provides, in a single administration, a patient's daily dose requirement of optimally effective methylphenidate, eliminating the need to take a second dose, while minimizing undesirable side effects and maximizing ease of administration.